Tools and Guidance for Creating Site-Specific Numeric Water Quality Criteria for Nutrients Available

As numeric nutrient criteria are developed by various states, the United States Environmental Protection Agency (U.S. EPA) has defined three categories of approaches: 1) Reference Condition Approach, 2) Stressor-response Analysis, and 3) Process-based (Mechanistic) Modeling, and has provided specific guidance for the first two approaches. One of WERF’s current research efforts is focused on the third “modeling” approach. The project team is developing case studies, decision support tools, and a Nutrient Modeling Toolbox for deriving site-specific water quality criteria for nutrients (LINK1T11). This model-based approach can be used alone or in combination with the other methods and may be particularly useful when the permit writers and permittees are faced with understanding the relative contributions from multiple sources. It is our goal that the WERF research will be used as a starting point of specific guidance for processbased modeling.

Strong science is the hallmark of the Clean Water Act. Under Section 304(a) (1) of the Act, the U.S. EPA is directed to develop water quality criteria that is effects based; reliant on all aspects of water quality – chemical, biological and physical, process for assessing concentration and dispersal of pollutants; and on the effects of pollutants on response variables such as biological community diversity, productivity, and stability, including eutrophication and rates of organic and inorganic sedimentation for various water bodies.

The focus of WERF’s Nutrient Modeling Toolbox is centered on the examination of existing models to meet Congressional direction in Section 304(a)(1). It also focuses on scientifically defensible methods for using all forms of scientific water quality data and information reflecting chemical, biological, and physical effects.

The Nutrient Impact Modeling Toolbox provides a mechanism that can quantitatively link nutrient loads to water quality and ecological response indicators on a site-specific basis. This Toolbox is designed to help local governments, utilities, states, and other stakeholders select modeling approaches based on a variety of factors. These factors include:

Regulatory applications associated with the various modeling options for derivation of water quality standard criteria.

Total maximum daily load allocations and effluent limits for point sources.

The models also consider targets for nonpoint source best management practices, and data and information for nutrient trading.

The Nutrient Modeling Toolbox contains information and access to approximately 30 models including process-based load-response models, empirical-based load-response models, and stressor-response submodels used to create hybrid models. There is an information sheet on each model, and all models presented in this Toolbox are publicly available. The models vary in simplicity and data requirements, and relatively modest resources can be brought to bear at an initial screening level to obtain preliminary results. This will help some users determine if a more rigorous data collection effort is warranted.

Within the Nutrient Modeling Toolbox, a Model Selection Decision Tool will guide users to find the most appropriate model(s) for their situation. The Selection Tool begins with a model classification matrix that considers waterbody type and response endpoint(s). It further refines model selection with questions on model application, dimensionality, temporality, and other secondary factors.

Ultimately, the goal for these modeling applications is regulatory decision making. Over half of the states currently have no formal review process. Fourteen states have some type of protocol such as guidance, review panel, or modeling quality assurance planning (QAPP) requirements. Six states have a list of approved models and some states are not yet using models.

Interaction with regulators at every step of the Nutrient Modeling Toolbox process is recommended. By involving regulators at project initiation, prior to model development and prior to model application, it’s possible to ascertain existing protocols for regulatory review of models, problems, QAPP requirements, and to reconcile differences between model results and regulatory frameworks.

To ensure communities are familiar with WERF’s Nutrient Modeling Toolbox, WERF has collaborated with the Water Environment Federation, National Association of Clean Water Agencies, and Association of Clean Water Administrators to inform U.S. EPA, state regulators, and end-users on the appropriate application of the Nutrient Modeling Framework and Toolbox. Look to the various websites for information about web seminars and other educational opportunities. The partner associations’ websites also contain additional information on web seminars, educational opportunities, and a link to download the Nutrient Modeling Toolbox at no charge.

To help ensure the usefulness of the Toolbox, the research team’s approach included review by a Stakeholder Advisory Panel from various agencies that could ultimately benefit from its implementation.